So this is my first guide but I do play ahri quite a bit and feel that this is a pretty solid build for midding with her

As far as runes go, they are pretty self explanatory. The ap quints and magic pen marks allow you to harass harder in the beginning of the game while the defensive seals and glyphs help you survive

For ahri I tend to go for as much ability power as I can get my hands on, but in order to do that you need to start slow. Once you have enough for a catalyst, go back and pick it up. It will make your harassing much much easier. ROA adds to the survivability and then of course once you get rabadans things are up hill from there. I usually choose merc treads because theres a pretty good chance that you will have at least 2 ccs on the other team.

What I try to do with ahri is hit them with the very end of my Q so that it does full damage without giving them the chance to move out of the way when it comes back. Also, you will want to combine youre E with Q and W to get maximum damage when you burst. Ahri is also an excellent diver because of her ult, so dont be afraid to turret dive with her if you know you can get the kill just pop W and ult your way past the turret for the kill.